This invention generally relates to DC voltage power supplies and more specifically is directed to a power supply for providing a well-regulated DC voltage output on a standby basis when a primary DC voltage source is removed or no longer available to drive a load.
DC power supplies capable of providing a well-regulated output voltage have widespread application, particularly in driving a wide variety of electronic devices. These DC power supplies typically include a conventional switching voltage regulator comprising an electronic switch such as a transistor and a comparator for comparing the output voltage of the regulator with a reference voltage and for turning off the switch when the output voltage exceeds a predetermined value. The output voltage is generally developed across a filter capacitor for providing a more constant, or level, DC output voltage. This type of switched-mode power supply is somewhat inefficient in that it typically utilizes a pulse width modulator which requires a large current for controlling the operation of the electronic switch.
One area in which low voltage DC power supplies are finding increasing utilization is in energizing microcomputer systems. These systems typically include a volatile memory and sometimes a nonvolatile memory of the complementary-metal-oxide semiconductor (CMOS) type which have extremely low standby power dissipation while requiring a high level of voltage regulation of the DC inputs provided thereto. Variation in the DC inputs provided to the microcomputer can readily affect the sequence of logic operations in the microcomputer so as to render the results thereof useless. In addition, in the case of a volatile memory, the contents thereof may be lost and thus unavailable for subsequent use if the DC input power drops below a predetermined voltage level. This type of power outage may be due to any one of a large variety of causes, perhaps the most common being due to short-term power line dropouts which occur, for example, when lightning strikes near a power line.
One approach to providing standby power in the event of a power loss involves the use of a power storage element such as a capacitor or a battery. While relatively small value capacitors may be used as power storage elements where the load device requires relatively small amounts of operating power, large value capacitors are needed in systems having digital signal processing devices which require relatively large amounts of operating power. As system power requirements increase, so do the size of the capacitors needed to provide the increased energizing voltages. The battery backup power supply system of the prior art has been undesirable as the batteries require frequent replacement or recharging circuitry which increases the complexity of the system. Moreover, batteries and relatively large value capacitors tend to be expensive.
A system for providing backup power for a memory element is disclosed in U.S. Pat. No. 4,227,257 to Sato wherein a second output terminal connected directly to the main power supply for powering the memory element is provided. Also provided are a capacitor for providing power to the second output terminal during interruptions of the main power source, a Zener diode for regulating the voltage of the second output when a power switch is turned off and a pair of diodes for allowing the second terminal to be driven by a voltage regulator output during normal operation and for preventing the second output terminal from driving the remaining circuitry when the power switch is turned off.
U.S. Pat. No. 4,249,089 to Wolford discloses a system wherein memory means are provided for storing a binary signal having first and second levels corresponding respectively to the "on" and "off" operating conditions of the system during the presence of operating power. Storage means are coupled to the memory means for selectively developing an enabling signal in response to the first level and restoring the enabling signal for a predetermined time period after the start of power dropouts. Switching means coupled to the memory means is rendered operative at the end of power dropouts to set the binary signal to its first level if the enabling signal output of the storage means is still present and to otherwise set the binary signal to its second level for rendering the system in an "off" operating condition. This system utilizes complicated control circuitry for detecting primary power source outage and switching to the backup supply.
U.S. Pat. No. 4,225,792 to Fahey discloses a detector circuit for detecting the loss or reduction of DC power to a DC load and for detecting a low battery voltage condition of a standby battery employd to power the load upon loss or reduction of DC power supplied to the DC load. The DC voltages utilized for energizing the DC load and charging the battery are derived from a DC power supply circuit with which the detector circuit of this invention is used. The detector circuit includes a transfer circuit responsive to the DC voltage provided to a load for selectively coupling either a primary DC power supply or a standby DC power supply to the load in response to the output voltage of the primary DC power supply. This circuit represents a complicated approach to providing backup DC power in the event of primary power source outage.
The present invention is intended to overcome the aforementioned limitations of the prior art by providing a switching regulator power supply which automatically switches to a backup state when primary DC power is lost and automatically turns off when primary DC power is reinstated. Thus, the present invention utilizes less power, eliminates RFI during periods of primary DC power supply operation, is particularly adapted for low current applications, and reduces the voltage requirements and prolongs the useful lifetime of a storage battery used as the backup DC power supply.